Electronic arc-suppressor



Jam. 13, 1959 wIH; HICKOK 2,868,941

ELECTRONIC ARC-SUPPRESSOR 7 Filed March 11, 1958 2 Sheets-Sheet 1 7 Jan.13, 1959 w. H. HICKOK ELECTRONIC ARC-SUPPRESSOR Filed March 11, 1958 2Sheets-Sheet 2 United States Patent 2,$68,9 l-ll ELECTRONICARC-SUPPRESSOR Willard I-I. Hickok, Louisviile, Ky., assignor to tChemetron Corporation, a corporation of Delaware Application March 11, 1958,Serial No. 720,698

9 Claims. (Cl. 21.9-10.77)

This invention is concerned with methods of, and systems for minimizingor preventing the formation of arcs between the heating electrodes ofhigh-frequency heating systems of the type particularly adapted for theheating of dielectric work.

The problem of protecting the work and the circuit components againstdamage due to the formation of arcs between electrodes is not new.Nevertheless, there has been lacking, until the present invention, asystem which acts in response only to incipient or actual arcingconditions with sufiicient speed and reliability to overcome costlyarc-damage and at the same time is insensitive to starting transientsand also insensitive to the ripple voltage present in a three-phaseunfiltered anode supply system. Though generally applicable tohigh-frequency heating systems and to high-frequency sources oroscillators, the importance of the present invention will be bestappreciated by reference to its application to a particularhigh-frequency heating system.

Dielectric heating systems are particularly adapted to the molding,curing, embossing and other processing of plastic materials. Forexample, one or both of the heating electrodes may comprise a die havingon its face a simple, but more generally a fairly complex, design to beimparted to the plastic material. Dies which include patterns areexpensive, and since thermoplastic materials faithfully reproduce theindentations, protuberances, etc., any impairment or damage to suchsurface will result in unacceptable final products. When the productsare molded panels for automobiles or other components used in assemblylines, not only is the loss of material substantial, but also importantis the lengthy shut-down time required to repair or replace the diesunless protective means, such as is the subject of this invention, areused.

Accordingly, it is highly important to minimize the formation of arcswhich may not only damage the work but which also burn or otherwise marand impair the surface of the dies. The minimum protection needed is toextinguish any are which may form before it can damage the work or thedies.

Protective systems must be able to distinguish between a genuine arc orincipient arc and other disturbances in the entire equipment to which itis applied. For example, the ripple frequency from an unfiltered powersupply, or the starting transient which may occur when an equipment isturned on, may cause false operation of the protective system thusshutting down the equipment needlessly. In the past it has been foundthat to avoid such needless operations, the sensitivity of theprotective system will be reduced by the user of the equipment to thepoint of failing to protect the die. In the system herein described thisproblem is eliminated by making the protective system relativelyinsensitive to such extraneous signals which may cause false operationswith out impairing the sensitivity to the damage-causing arc signals.

As explained in Jennings and Smith application Serial No. 544,856, filedNovember 4, 1955, and assigned to the same assignee as the presentinvention, it has been found that losses due to arcing can .beavoidedalmost entirely by providing a system which in response'to an abnormalrate of change in the high-frequency voltage of the load circuit actswith such speed that arcing does not occur to any consequential degree.The abnormal rate of change of the high voltage of the load circuit isindicative of incipient or actual arc formation.

in accordance with the present invention, a protective system isprovided which is insensitive to extraneous signals such as startingtransientsand ripple frequencies from the D. C. power-supply but isquite sensitive to transients or signals representative of incipient oractual arcing conditions. The protective system of the present inventionincludes two sensing means, each having amplifiers with input and outputcircuits. The input circuit of one amplifier is connected to the directcurrent power supply circuit through an impedance element so that thefirst sensing means responds to voltage variations which occur in thedirect current power supply circuit. The second sensing means has itsinput circuit coupled to the hgh-frequency heating circuit. There isincluded in the second input circuit a rectifying means so that'thesecond sensing means likewise responds to voltage variations but thistime those occurring in the heating circuit.

Both the direct current power supply circuit and the heating supplycircuit include the starting transient and the ripple voltage. Whenarcing conditions occur, an arc transient appears only in the heatingcircuit. By connecting the output circuits of both sensing means to abalancing circuit and in phase opposition, that is with oppositeinstantaneous polarities, the ripple voltage and the starting transientsmay be balanced out leaving only the arc transient when it occurs.

Due to the effects of the intervening circuitry, it may be that thestarting transient whch appears on the heatin" electrodes is not exactlylike that occurring in the power supply although it originates in thelatter place. Furthermore adjustments or changes in electrode voltagemay alter the relative magnitudes. Therefore it may be that the startingtransient is not completely balanced out. Since the starting transientis normally of one polarity, a third amplifier may be coupled to animpedance element in the balancing circuit so that it will only respondto a voltage change of opposite instantaneous polarity to that arisingdue to the starting transient. Accordingly, by reason of the two sensingcircuits, and the balancing circuit, there may be produced an outputsignal from the third amplifier representative solely of arcingconditions. This third amplifier may be provided with high gain so thatthe signal representative of arcing conditions may be greatly amplified.By reason of the above described combination there may be utilized asensitivity of an order several times greater than heretofore has beenfeasible. Accordingly a more favorable signal-to-noise ratio isattained.

In response to a signal representative only of arcing conditions, thesupply of highfrequency power to the heating electrode is interrupted toprotect the work and components of the system.

For further objects and advantages of the invention, reference may behad to the following detailed description taken in conjunction with theaccompanying drawing in which there has been schematically illustrated ahighfrequency heating system embodying the protective system of thepresent invention.

Referring now to the drawings, Fig. l, the invention in one form hasbeen shown as applied to a high-frequency dielectric heating system,including heating electrodes 1t] aseaaal and 11 arranged in a spacedrelation to accommodate between them a load 12. The load, which maycomprise work pieces of dielectric material, is heated when sub jectedto the high-frequency, high-voltage electric field produced between theelectrodes and 11. Inasmuch as the heating of the load increases as thesquare of the voltage between electrodes 1i) and 11, it is desirable tohave that voltage as high as possible to shorten the length of time theload is between the electrodes to bring it to a desired temperature orto a desired condition of dryness. However, as the voltage across orbetween the electrodes 10 and 11 is increased, there is the probabilityof an occasional are between the elec trodes. Even if the voltagesbetween electrodes it} and 11 be reduced to or maintained at a valuewhich will normally be safe from the arc prevention standpoint, arcs maybe formed. For example, if, as loads 1?. are changed, foreign objectsare brought between the electrodes, arc damage may occur. Such foreignobjects may comprise iron fillings, metal staples, tacks or other partsused in the fabrication of automobile trim panels and the like. If anarc should form between electrodes 10 and 11, there will be burning ordegradation of the load 12 disposed between them; and where electrodedies are employed, there may occur a pitting of them which may renderthem useless for further application and use.

Before describing the protective system itself, brief reference will bemade to the power generator or source of high frequency which may betaken as representative of typical power generators. They may takevarious forms and may include one or more stages. As shown, thehigh-frequency source is of the T. N. T. type. It includes a power tube13 of the thermionic type having the customary electrodes therein. Ithas a tank circuit comprising an inductance 14 and a capacitor 15. Thetank circuit is connected to the anode of power tube 13 by way of acoupling capacitor 16. A variable condenser 17 is included in thehigh-frequency power output circuit and is utilized for coupling theheating circuit to the tank circuit. A filament transformer 18 providesthe heating current for the filament of power tube 13. Thegrid-excitation circuit includes a grid inductance or coil 20. Thegrid-excitation circuit also includes a resistor 21 and an R. F. choke22. The direct current power supply circuit for the oscillator includesa rectifying means 24 energized from alternating current supply lines25. As shown, the rectifying means 24 is supplied from 3-phase supplylines. The rectifier itself may be of the full-wave type, ofconventional design. As a result, a ripple frequency of 360 cycles persecond will be present in the D. C. supply to tube 1.3. If singlephasealternating current were utilized, the ripple fre quency would be 120cycles per second. The direct current power supply circuit includes ahigh-frequency choke 23.

To start a heating cycle, a starting switch 3t) is momentarily closed toenergize, from source B, the operating coil 31 of a contactor 32 whichcloses the circuit from the alternating current supply lines 25 to therectifying means 24. The contactor 32, upon closing, completes a holdingcircuit through contacts 33. Upon closure of the contactor 32, power issupplied to the power tube 13 and high-frequency alternating current issupplied to the heating electrodes 10 and 11 for the treatment orheating of the work-load 12. The power supply may be deenergized bypressing a stop button 34 which deenergizes contactor 32.

The sudden application of energy by closing contactor 32 applies atransient to both the rectifier means 24 and the electrode systems 10and 11. These transients, which appear when starting the equipment have,in the past, given a signal to the protective device causing it tofalsely operate. Furthermore, the ripple voltage appearing upon the highD. C. voltage applied to the plate of tube 13 appears as a modulation ofthe high-frequency voltage at the electrode system 141* and 11. Both ofthese effects, i. e. transients and ripple as well as sudden linevoltage changes set the maximum sensitivity at which prior artprotective devices could be operated in practice. In order to eliminate,or greatly minimize these factors limiting the sensitivity of theprotective device a circuit, in accordance with the present invention,is described consisting of two sensing means each including amplifiers37 and 3?. The amplifier 37 includes an input circuit which isnon-conductively coupled to the D. C. power supply circuit as by way ofa capacitor 42, an R. F. choke 43, a voltage divider including apotentiometer 44, capacitors 45 and 46, and resistor 47.

The amplifier 37 includes a grid-leak resistor 49 connected between tworesistors 56 and 51 in the cathode circuit. The resistor 53 supplies theproper bias for the grid circuit. The direct current power supplycircuit for the amplifier 37 extends from the terminal labeled l3 by wayof a plate resistor 52, the tube which has the reference character 37and by way of the resistors 50 and 51 to ground. As shown, the outputcircuit for the amplifier 37 is taken between ground and the upper endof resistor Stl by way of a coupling capacitor 53 to one side of asingle-pole, double-throw switch 54. The signal at the cathode outputcircuit is an exact replica of the signal derived from the directcurrent power supply circuit extending from the rectifying means 24.

If the switch 54 be moved to its upper position, then an output circuitwill extend from the anode by way of a coupling capacitor 55. The outputsignal will then be an inverted replica of the derived signal from thepower supply circuit from rectifying means 24.

By moving the contact 44a of potentiometer 44, the amplitude of eitheroutput signal appearing at the switch 54 may be adjusted as desired. Theoutput signal at switch 54 is applied to a potentiometer 557. Thepotentiometer 57 is an impedance element in a balancing circuit whichincludes the output circuits of both sensing means, only one of whichhas been thus far described.

The second sensing means including amplifier 38 has an input circuitwhich includes a non-conductive coupling element such as capacitor 6hconnected between the two resistors er and 62 of a voltage divider. Anelectrode 65 spaced from heating electrode 10 forms with it a capacitorwhich capacitor in conjunction with a capacitor 64 forms a capacityvoltage divider. Rectifying means shown as a diode 58 by-passes positivepulse and applies by way of resistor 59 and capacitor 65 anegatively-poled direct current voltage to the voltage dividers 61 and62. Voltage changes appearing across resistor 62 are applied to theinput circuit of the amplifier 38 by way of the capacitor 60. Theamplifier 38 is provided with a grid-leak resistor 66 connected betweencathode resistors 67 and 68. The amplifier 38 has an output circuitillustrated as extending from the upper end of cathode resistor 67 byway of a coupling capacitor 69 to a single-pole, double-throw switch '70which is connected to the potentiometer 5'7. An exact replica of voltagechanges, as derived by way of the capacitor formed between electrodes orplates 10 and 63 will appear at the output circuit of amplifier 38.

If the single-pole, double-throw switch 7% be moved to its upperposition for completion of an output circuit by way of capacitor 71 tothe anode, an inverted replica of the derived signals will be obtained.

As illustrated, the signals appearing at the two output circuits areapplied to the balancing circuit including the potentiometer 57 inopposition one to the other. Normal voltage and circuit-componentsize-deviations may be compensated for by adjustment of either contact57a on potentiometer 57 or by contact 44a on potentiometer 44.

With signals ofequal amplitude applied to the grids .of the twoamplifiers 37 and 33, contact 57a may be adjusted to producesubstantially zero output by way of that contact. In order to establishequality of the signals at the respective grids or input circuits ofamplifiers 37 and 38, the contact 44a will be adjusted to the properposition for that result. These contacts 44a and 57a also permitoperation short of balanced conditions or with over-compensation.

The means by which similar signals from the power supply and the heatingelectrodes, such as power supply ripple, line voltage fluctuations,power supply transients, and the like are balanced out, so they will notoperate the protective system, has been described. An arc in theelectrode system will produce a signal in the amplifier 38 but will notproduce a like-signal in amplifier 37. Then, since there is no balancingsignal, a substantial signal due to the arc will appear at the contact57a of the potentiometer 57. This signal is subsequently amplified bytube 75 as described below.

Inasmuch as spurious transients or signals which are not indicative ofarcing or incipient arcing appear at both sensing means, the result ofbalancing out such signals at the balancing circuit indicates thatsubstantial amplification may be provided for the arc-indicating signalswhich do appear at the output circuit connected to potentiometer contact57a. Thus, that output circuit applies signals to the input circuit ofan amplifier 75 shown as a pair of triodes connected in parallel. Theamplifier 75 is connected to a source of anode supply, as from theterminal labeled 3 and to have the negative terminal labeled B 1 Thistube is operated in a highly conducting fashion, i. e. practically nobias is used. Resistor 77 is used to limit the grid current whichnormally flows. Operated in this fashion this tube will accept onlynegative polarity pulses. The input circuit to the amplifier 75 includesthe resistor '76 and the grid resistors 77.

The output circuit from amplifier 75 extends from the anode side ofplate resistor 78 by way of a coupling capacitor 79 to the input circuitof a high-speed circuit controlling arrangement. This arrangementincludes a thyratron 30 having its grid-control circuit connected to asource of bias voltage 81 so as to be normally non-conductive. Thesource 81 is connected by way of a series resistor 82 to a potentiometer83 from which a desired bias voltage is selected by contact 83a. Thebias voltage is applied by way of the coupling resistor 84 to the gridof the thyratron 8d. The anode supply for the thyratron 89 is derivedfrom a direct current source whose supply contact has been labeled B t.It is to be noted that the capacitor 86 is charged during the time thethyratron 80 is non-conductive. It is to be further observed thatcurrent may flow, while thyratron 80 is non-conductive by way ofresistors 37 and 99, a rheostat 91, conductor 2, operating coil 950 of ahighspeed vacuum-type grid-relay 95, and thence to the other side of theB supply, B The current flow by way of the relay coil 956 isinsufiicient to operate the relay contacts to their open position. Withcurrent flow through the relay winding 950 of a value just insufiicientto operate the contacts 95a to their open position, a much greater speedof opening of the contacts is attained upon the firing of the thyratron'85.

The high-speed vacuum switch or relay 95 is actuated to open the gridcircuit of the power supply tube 13 upon the appearance of anypositive-going pulse at the grid of thyratron 8d. The opening of thegrid circuit of the power supply tube 13 immediately interrupts thesupply of high-frequencypower to the heating electrodes and 11. Thisaction occurs within a few milliseconds after pulses indicative-ofarcing conditions occur. In order to attain maximum speed of operation,the capacitor 86 aids in increasing the discharge current throughthyratron ht once it is fired, and assures alarge rush of current-through winding 950 for maximum speed of opening of the vacuum switch95. If desired, the vacuum switch 95 may have additional contacts 95];arranged to be opened with energization of operating coil 95:. Theopening of the circuit through contacts 95b will deenergize the coil 31of contactor 32 which will then open.

In this connection, it is to be noted that contactors and magneticswitches are generally slow in operation as compared with speciallydesigned high-speed relays of the type utilized for the vacuum switch95. Such relays are available on the market, as relay type VS-Z fromEitel-McCullough Company.

With contacts 951) open, the contactor 32 will move to its openposition. Operation of the high-frequency heating system may be resumedby operating the stop button 34 to reset the protective system and thenthe starting button 36. It will be noted the stop switch 34 is providedwith contacts 34a in the discharge circuit of the thyratron 80. Thus, assoon as contacts 34a have been opened, the thyratron 86 will bedeenergized. While deenergized, the capacitor 86 will again be chargedpreparatory to a second protective operation in the event anarcindicative input signal is again applied to the input circuit ofthyratron 80.

Summarizing the operation of the system as a whole, when the startingswitch 30 is closed and the contactor 32 closes to energize therectifying means 24, starting transients appear both in the directcurrent sensing circuit and in the high-frequency sensing circuit.Similarly, the ripple voltage, 360 cycles per second for the 3-phasefullwave rectifier, appears in both of the sensing circuits. Thus bothsensing means including amplifiers 37 and 38 have present the startingtransients and the ripple voltages. These are balanced one against theother so that notwithstanding the fact there are signals present in bothsensing means, nevertheless, properly adjusted, there is approximatelyzero output from the output of the balancing circuit, including thecontact 57a and resistor 76. Accordingly, amplifier may have high gainto provide great sensitivity in respect to transients which appear inbut one of the sensing means. Thus, arcing conditions between electrodes10 and 11 produce a transient of high magnitude which results in asignal of substantial magnitude at the input of amplifier 38 and at theamplifier 75. By reason of the highly amplified signal, the thyratron 80is more reliably fired to interrupt the supply of highfrequency power tothe heating electrodes in case of an arc.

in practice, it has been found that the balanced sensing means of thepresent-invention has been highly reliable in operation indistinguishing between (1) ripple frequency, (2) starting transients and(3) other spurious fluctuations and transients due to arcing conditions.Down time has been decreased and the system as a whole is operated withmuch greater reliability. Furthermore the sensitivity to arcs has beenmaintained high so that loss of dies and load material has beensubstantially reduced.

Another feature of this invention is that the protective system is readyto function before the power is turned on. Thus, if there is a fault orarcing condition present at the start of the heating cycle as would bepresent if there were metal inclusions in the load material, asmentioned above, immediate protection is available to stop oscillationsbefore damage can be done.

The switches 54 and 70 provide great flexibility in setting up thesystem in that they can be operated from one position to the other toassure balancing out the signals in the two sensing circuits, as in thebalancing circuit including potentiometer 57. Thus with groundedanode-supply circuits the polarities or relative phases of the signalsmay be reversed by these switches.

While the adjusting means including the potentiometers 44 and 57 havebeen described, it will be convenient in many systems to provideadditional features for ease in setting up the system for particularfield conditions. For

example, a magic eye indicator tube 105 has been shown as connected tothe output of the amplifier 75. In the absence of any signal therefrom,the target or eye is wide open. However, when a signal appears at theoutput of amplifier 75, it will be applied by way of a couplingcapacitor 106, a resistor 107, a rectifier 108 and a resistor 109shunted by a capacitor 110, to the input circuit of the indicator tube105. The output signal of the amplifier is thus rectified and theresultant voltage as pearing at the input or grid circuit of tube 105causes the eye or target partially to close. The degree of closure willdepend upon the amplitude of the signal from the amplifier 75. It is tobe noted that the filament of the tube 105 is supplied by way of atransformer 111 energized from alternating current supply lines. Thefilament supply transformer 111 not only energizes the filament of tube105 but energizes in series therewith the filament of diode-detectingtube 58. If this rectifier tube 58 should fail by reason of filamentfailure, the green fluorescence of the indicator tube will disappearthus providing visual indication of failure of tube 58.

If, with the power tube 13 energized, the eye of the indicator tube 105is partially closed, the contact 4441 of potentiometer 44 will beadjusted for maximum opening. Thus the indicator tube 105 provides aneasy way of making one of the initial adjustments for the system.

The following typical values of the circuit components for oneembodiment of the invention are to be taken as illustrative and not byWay of limitation:

Tubes 37, 38 Type GL5692. Tube 75 Type GL5691. Tube Type 6012. Tube Type6E5. Capacitors Value 42, 53, 55, 60, 69, 71 and 106 .05 64, 65 .0002 79.02 0.001

Resistors Value 44, 61, 77 1 100 49, 62, 66, 76 meg l 50, 67 "K" 1 51,68 K 10 52, 52A K 57 meg 0.5 59 K 20 84 1 330 107 "K" 270 109 meg 4.7

The remaining circuit components are conventional and have conventionalvalues adapted for the particular type of tubes utilized.

It is to be understood that the present invention is applicable tohigh-frequency generators of many types currently utilized inhigh-frequency heating. They may be of the type illustrated in saidJennings et a1. application, Serial No. 544,856, of the type shown inWilson Patent No. 2,684,433 and in Warren Patent 2,783,344. They may beprovided with grounded positive circuits or, as shown in Fig. l, withgrounded negative circuits. For any of the various systems to which theinvention may be applied, it will be found that adequate flexibility hasbeen provided for operation in conjunction therewith.

In Fig. 2 there has been illustrated the present invention applied to asystem for interrupting the supply of high-frequency power to theheating electrodes by developing a high negative bias in the gridcircuit of the power supply tube 13. A part of the system is like aportion of the system of Fig. 1 of aforesaid application S. N. 544,856.

In Fig. 2, the output from the amplifier 75 is applied by way ofcapacitors 112 and 113 to the control circuit of a thyratron 114. Asshown, the first thyratron 114 is used to control a second thyratrontype of tube 125.

By providing a trigger circuit including the tube 114 and associatedcircuits, a control pulse or signal of adequate magnitude is produced tofire the tube 125, which is of a type requiring a large pulse to renderit conductive.

The tube 114 may be either of the GL5727 or 2D21 type, both beinggas-filled and the GL5727 having an extra-duty rating. Whenever a signalpulse from the balanced sensing means appears at amplifier 75, it isapplied by way of the coupling capacitors 112 and 113 between the gridand cathode of tube 114. The input circuit also includes a grid resistor115 and a cathodebiasing resistor 116, shunted by a regulator type oftube 117 of the neon-tube type. The anode and shield grid of tube 114are connected respectively through resistors 13 and 111 to the positiveor 13+ conductor of a suitable source of anode potential. A normallynegative bias potential for the grid of tube 114 is developed by avoltage-divider including resistors 116 and 119. The circuit throughsaid resistors extends from B-}- by way of resistor 119, the conductiveconnection to the cathode, the bias resistor 116 and by way of resistorto B. The potential difference developed across resistor 116 maintainsthe grid of tube 114 negative with respect to its cathode and thusmaintains the tube non-conductive until there is applied to its inputcircuit a positive signal pulse from amplifier '75.

During normal operation, the neon-tube 117 is nonconductive since thepotential difference, of the order of 6 volts, across resistor 116 isinsurTicient to render tube 117 conductive. When the tube 114 fires, thevoltage across resistor 116 rises and tube 117 is rendered conductive toprovide a discharge path of decreased resistance through the tube 114and resistor 1.20.

Upon firing of the tube 114, a surge of current flows through it andthrough resistor 120 by reason of the additional provision of acapacitor 121 which carries a charge equal to the potential differenceor voltage of the B supply. The resistor 118 has a sufficiently highresistance to limit the current fiOWing through the circuit, includingresistor 118, the tube 1.14 and resistors 116 and 120, to a value whichwill be insufficient to maintain ionization of the tube 114.

Accordingly, as soon as the capacitor 121 has discharged, currentflowing by way of resistor 118 is insufiicient to maintain the tube 114conductive. As it and tube 117 return to their non-conductiveconditions, current flows by way of resistor 118 to recharge thecapacitor 121 to condition the trigger circuit of the protective systemfor a second operation.

The principal resistance in the discharge path is resistor 120. It has avalue for development of a highvoltage impulse, well in excess of 130volts. The highvalued impulse is applied by coupling capacitor 122 tothe input circuit of the second control tube 125, which in oneembodiment of the invention was of the AX-9911 type, an extra-duty 4C35thyratron tube.

Though a permanent source of anode voltage may be provided for tube 125,such as from a conventional rectifier system, the system of Fig. 2utilizes a source of stored electrical energy such as one or morecharged capacitors. Thus when tube 125 fires, electrical energy storedin capacitors 126 and 127 produces current flow through the gridresistor 128 of tube 13 of magnitude and direction to bias that tube toand beyond cut-off.

' As a result, the supply of high-frequency power to the heatingelectrodes 10 and 11 is instantly interrupted. In this manner the powertube 13 itself serves the function of interrupting the supply to thehigh-frequency load circuit including electrodes 10 and 11. Thus, thepower tube 13, an electronic discharge device, is utilized not only forthe delivery of high-frequency power to maintain high-frequencyoscillations within the tank circuit, but it is utilized also as anelectronic switch or circuitinterrupter to shut off the power to thetank circuit upon the occurrence of incipient or actual arcingconditions.

The tube 13 also interrupts the direct-current anode supply circuitwhich includes the R. F. choke coil 23.

The discharge circuit for the capacitors 126 and 127 may be traced fromthe positive side thereof by way of resistor 129, conductor 130,grid-leak resistor 128, and by way of the thyratron 125 to the otherside of them. Because the energy-storing means comprising capacitors 126and 127 represents a temporary source of supply, the tube 13 ismaintained nonconductive by the negative bias developed by resistor 128only during the period of adequate current flow through that resistor.With a permanent source of supply, the tube 13 can be maintainednon-conductive as long as may be desired.

To provide continued protection against an immediately recurring arc,which might appear if the heating cycle were to be resumed as capacitors126 and 127 are dis- .charged, additional provisions are made tomaintain in terrupted the supply of high-frequency power to theelectrodes and 11 after discharge of the capacitors. As will be laterexplainedin detail, the relay or contactor 132, normally closed bycurrent flow through its operating coil and a tube 133, is deenergizedand operates to its open positionduring the time tube 13 is maintainednon-conductive by the discharge current supplied by capacitors .126 and127.

A more detailed consideration of the circuitry including the capacitors'126 and 127 will now be presented. The energy stored in the capacitors126 and 127 is sup plied by a charging circuit which includes aseries-resistor 135, a filtering capacitor 136 and a'rectifier or diode137. The diode 137 is supplied from a transformer 138 having the usualfilament windings and an anode supply winding connected in the chargingcircuit. This circuit may be traced from the left-hand side of theanode-supply winding by way of the resistor 129, the capacitors 126 and127, the resistor 135 and by way of the diode 137 to the other side ofthe anode-supply winding.

The series resistor 135 has a value which limits the magnitude of thecharging current to a value which will not maintain the thyratron 125conductive. Accordingly, after it fires andthe capacitors 126 and 127have been substantially discharged, the thyratron 125 ceases to conduct.As soon as it becomes non-conductive, the charging circuit is effectiveto initiate the re-charging of the capacitors 126 and 127. The relay 132is maintained in its open position until sufiicient energy has beenstored in the capacitors to assure effective protection of the system inthe event of incipient arcing upon resumption of the high-frequencyheating cycle.

Returning now to the operation of the relay 132, it is normallyenergized to close the anode-supply circuit of power tube 13. The coilof relay 132 is energized by reason of the flow of current from thesource of supply B-} by way of conductor 141, the operating coil of therelay 132, conductor 142, the tube 133, which may be one-half of aheavy-duty 6SN7 type of tube, and thence by way of resistor 143 toground, which is the other side of the B supply.

The tube 133 is conductive in the absence of current flow through aresistor 144 since the grid is not then biased to cut-off. Morespecifically, in the absence of current flow through resistor 144, thegrid of tube 133 will be at the same potential as its cathode, acondition for conduction by tube 133. The magnitude of current flowingthrough tube 133 is determined by voltage-dividing resistors 145 and143. These resistors determine the potential of the cathode of tube 133relative to the anode. The current flow through the coil of relay 132and the tube 133 is limited to a value which will close the relay 132and which will hold it in closed position.

The tube 146 has its grid negatively biased relative to its cathode byreason of the grid connection to the tap 147 of the voltage-dividerformed by resistors 148, 149, and 129, the latter resistor having itslower end connected to the ground path to which the cathode of tube 10.146 is directly connected. The resistor 129 is small com pared withresistors 148 and 149. Itis incorporated to accelerate thedeenergization of relay 132.

It will be recalled that when the thyratron fires, the discharge circuitincluded resistor 129. The potential difference resulting from currentflow through the resistor 129 makes its upper end positive relative toground. This cancels the negative potential between tap 147 and the topof resistor 129. The result is immediate removal of the negative bias onthe control grid of the tube 146. The removal of this negative bias fromthe input circuit of the tube 146 allows anode conduction of that tubeby way of resistors and 144. The resultant voltage drop across resistor144 makes the grid of tube 133 negative with respect to its cathode soas to reduce the flow of the current through tube 133. In this manner,the current is reduced below the hold-in value of relay 132 whichimmediately opens to interrupt the anode supply of the power tube 13.

There will now be described the manner in which the high-frequencyheating cycle is automatically resumed. As the capacitors 126 and 127are charged, the voltage across resistor 149 rises. it rises to a valuewhich biases the tube 146 to cut-off as the level of charge ofcapacitors 126 and 127 rises to a circuit-protecting level. As the tube145 is made non-conductive, current flow through resistor 14- is reducedto zero and the potential drop across resistor 144 disappears. Theresult is that the potential on the grid of tube 1133 again rises tothat of the cathode. The tube 133 conducts with flow of current toenergize the operating coil of relay 132 to reclose it and, in theillustrated embodiment, to initiate a further cycle of heating by thedielectric heating system. Alternatively, the system may be set up formanual initiation of such further heating cycle, as by suitablecontactors provided in supply means 24.

From the foregoing, it will be seen that upon occurrence of corona orflash-over, the system operates immediately to interrupt the flow ofpower to the load circuit. The operation is positive and certain byreason of the several stages of amplification and the large pulsesproduced which immediately shut off the power tube 13.

What is claimed is:

l. A protective system for a high-frequency heating system whichincludes heating means for a load, an oscillator supplyinghigh-frequency power through a power output circuit to said heatingmeans, and a power supply circuit for said oscillator for supplyingcurrent to said oscillator, comprising a first sensing means includingfirst amplifying means having input and output circuits, means forconnecting said input circuit to said power supply circuit for applyingto said first amplifying means signals representative of voltage changesin said power supply circuit, a second sensing means including secondamplifying means having input and output circuits, means including acoupling element connected to said power output circuit for applying tosaid input circuit of said second amplifying means signalsrepresentative of voltage changes in said high-frequency power outputcircuit, a balancing circuit including an impedance element and saidoutput circuits of said sensing means, said output circuits of saidsensing means being connected into said balancing circuit for applyingin opposing relation therein output signals having the sameinstantaneous polarities, a third amplifying means having an outputcircuit and an input circuit connected to said impedance element andresponsive to output signals developed by said impedance element, andmeans connected to said output circuit of said third amplifying meansfor interrupting the supply of high-frequency power to said heatingmeans in response to said output signals developed upon application toone only of said sensing means of an input signal.

2. The protective system of claim 1 in which said means connected tosaid output circuit of said third am- 11 plifying means comprises ahigh-speed switch for interrupting the supply of high-frequency power tosaid heat ing means, and electric discharge means operable in re sponseto an output signal from said third amplifying means for producinghigh-speed operation of said switch ing means.

3. The protective system of claim 1 in which said impedance element ofsaid balancing circuit comprises a potentiometer with a contactadjustable to vary in the balancing circuit the relative amplitudes ofthe signals from the output circuits of said sensing means for reducingthem to minimum values, and in which said means connected to said outputcircuit of said third amplifying means includes a thyratron having aninput circuit responsive to output signals from said third amplifyingmeans for rendering said thyratron conductive, and a high-speed switchoperable upon discharge of said thyra tron for interrupting the supplyof high-frequency power to said heating electrodes.

4. A protective system for a high-frequency heating system whichincludes heating electrodes for a load disposed therebetween, a poweroscillator tube for supplying high-frequency through a power outputcircuit to said heating electrodes, and a direct current power supplycircuit for said oscillator including rectifying means energized byalternating current for supplying direct current to said oscillator,comprising a first sensing means in cluding a first amplifier havinginput and output circuits, means including a coupling capacitor forconnecting said input circuit to said power supply circuit for applyingto said first amplifier signals representative of voltage changes insaid power supply circuit, a second sensing means including a secondamplifier having input and output circuits, means including a capacitorand rectify ing means connected to said high-frequency power outputcircuit for applying to said input circuit of said second amplifiersignals representative of voltage changes in said high-frequency poweroutput circuit, a third ampli fier having an output circuit and an inputcircuit, means including a potentiometer having a variable arm which isconnected into said input circuit of said third amplifier and whichforms in conjunction with said output circuits of said first amplifierand of said second amplifier a bal ancing circuit for combining inopposing relation therein the output signals from said first and saidsecond amplifiers, and means connected to said output circuit of saidthird amplifier for interrupting the supply of high-frequnecy power tosaid heating electrodes in response to output signals from said thirdamplifier.

5. The protective system of claim 4 in which said means connected tosaid output circuit of said third amplifier comprises a thyratron havingan input circuit with biasing means normally biasing said thyratron tobe nonconductive and upon application thereto of a signal from saidoutput circuit of said third amplifier being rendered conductive, andmeans operable in response to discharge current through said thyratronfor interrupting the supply of high-frequency power to said heatingmeans.

6. The protective system of claim 4 in which there is provided anindicating means having an energizing circuit connected to be responsiveto the output from said third amplifier whereby the ampiltude of thesignals applied to said balancing circuit may be adjusted to mini mizethe output signal from said third amplifier.

7. A protective system for a high-frequency heating system whichincludes heating electrodes for a load, an oscillator supplyinghigh-frequency power through a power output circuit to said heatingmeans, and a direct current power supply circuit for said oscillatorhaving rectifying means energized by alternating current for sup plyingdirect current to said oscillator, comprising a bal ancing circuitincluding a potentiometer, means for applying to said balancing circuitsignals derived from said direct current power supply circuit, saidmeans includ' ing a capacitor and an amplifier, means for applying asecond signal to said balancing circuit representative of voltagevariations at said heating electrodes, said lastnamed means including arectifier and an amplifier, means for adjusting the relative amplitudesof said signals applied to said potentiometer so that said signalsduring steady state conditions balance each other, and means responsiveto unbalanced signals from said potentiometer for interrupting thesupply of high-frequency power to said heating eelctrodes.

8. The protective system of claim 7 in which signal indicating means isconnected to said balancing circuit for indicating unbalance duringsteady state conditions whereby said amplitude adjusting means may beoperated to positions for a minimum output signal from said balancingcircuit.

9. The protective system of claim 8 in which said signaldndicating meansis an indicator tube having an electron beam and means including anamplifier connected to said balancing circuit for controlling said beamin accordance with output signals from said amplifier.

References Cited in the file of this patent UNITED STATES PATENTS

